1. Field of the Invention
The present invention relates generally to the field of electric motor controllers. More particularly, the invention relates to a technique for selectively connecting an electric motor to a variable frequency motor drive and a second electric power source.
2. Description of the Related Art
Electric motors are utilized in a number of different industrial applications. For example, a motor may be utilized to drive a conveyor belt used to transport materials. In some applications, it may be desirable to vary the speed of the motor. In the above example, it may be desirable to vary the speed of the motor so as to vary the speed of the conveyor belt and, consequently, vary the amount of material that is transported.
Typically, the speed of a polyphase induction motor depends upon the frequency of the electricity used to power the motor. Electric motors commonly are powered from a conventional power line, such as single-phase or three-phase AC operating at 60 Hz. Because these motors receive power at a single frequency, they typically operate at a single speed. Alternatively, motors may be powered by a variable frequency drive. A typical variable frequency drive receives power at a fixed frequency, e.g. 60 Hz AC, and produces a variable frequency output that is used to power the motor. By varying the frequency of the power to the motor the speed of the motor is varied.
Motor controllers that utilize a variable frequency drive to power an electric motor may have a bypass circuit to enable the variable frequency drive to be bypassed. Thus, instead of being powered by the output of the variable frequency drive, the electric motor may be powered by an alternate device connected to the input supply of the variable frequency drive. Bypassing the variable frequency drive enables power to the variable frequency drive to be removed but permits further operation of the motor while power is removed from the variable frequency drive. This enables maintenance to be performed on the variable frequency drive while the motor is operating. However, even with the variable frequency drive bypassed, voltage may still be present in or near the variable frequency drive. This voltage presents a safety hazard to a person attempting to perform maintenance on the variable frequency drive. There is a need, therefore, for an improved technique for bypassing power for a motor around a variable frequency drive.